This tip come from big wall expert Andy Kirkpatrick. Check out his excellent book on big wall climbing, “Higher Education.”
Yosemite valley in the summer can be a reflector oven on a big wall. If you find yourself in the heat, here's two easy ways to stay a bit cooler.
small folding umbrella
water mist/spray bottle
Between some instant shade and evaporative cooling, you can stay a lot more comfortable at that multi-hour belay.
And, that will give you some time to think about why most people like to climb in Yosemite Valley in the spring and autumn. :-)